This invention generally relates to the field of optical filtering and in particular to the field of integrated optic filters, their method of fabrication, and use in such applications as wavelength division multiplexing, beam conditioning, and spectral analysis.
Optical filters, whether passive or actively tunable, are well-known for their uses in controlling the amplitude and spectral content of radiant sources, spectral analysis, switching, modulation, and wavelength selection capability, especially as applied to wavelength division multiplexing in the field of communications where fine wavelength resolution produces high channel density.
A number of different technological approaches have been used in the design and implementation of optical filters to provide the required functionality for the above applications. Among these approaches have been those based on tunable liquid crystals (LCTF), tunable acousto-optics (AOTF), unbalanced Mach-Zender interferometry, reflective waveguide arrays, and many types of etalons.
The underlying principle behind the operation of some optical filters is multi-beam optical interference. Here, the optical path length of the individual light beams in the device is varied, after which the beams are interfered. Since the effective path length is wavelength dependent, it follows that the interference is wavelength dependent, which yields a wavelength dependent optical response. High quality filters based on this technology generate large optical path length differences, while maintaining fine control over them, and precise control of the interference points. For tunable filters, the effective path length must be adjustable through some external excitation.
Even though there are any number of devices that rely on interference phenomena, there is a substantial range in the techniques by which these interference effects are achieved. One of these is described in U.S. Pat. No. 5,353,317 to Weber which shows a massively parallel version of a Mach-Zender type filter where the paths are all separate waveguides. Another is shown in U.S. Pat. No. 5,526,439 to Bergman which uses couplers to interfere beams. Another coupler based tunable filter is shown in U.S. Pat. No. 4,390,236 to Alferness which describes an integrated optic device based on polarization rotation.
Other optical filters rely on polarization effects to achieve selective tunability. For example, U.S. Pat. No. 4,197,008 describes device in which spatially varying electric fields are applied to successive regions of a bulk optic crystalline medium disposed between a pair of suitably oriented polarizers to achieve filtering capability. U.S. Pat. Nos. 5,015,053, 5,002,355, and 4,223,977 all make use of two polarization guides and different methods of exciting them.
While there have been a variety of approaches to optical filtering that have been more or less successfully used in the past, there continues to be a strong demand, driven in large part by the requirements of the telecommunications industry, for tunable optical filters that can be easily fabricated and operated, and it is a primary object of this invention to satisfy the need for such devices.
It is another object of the invention to provide tunable optical filters in integrated optic format.
It is yet another object of the invention to provide tunable optical filters of small scale yet with fine control and coarse control over path lengths.
It is still another object of the present invention to provide tunable optical filters for use in telecommunications, beam control, and spectrometry applications.
Other objects of the invention will in part be obvious and will in part appear hereinafter in the detailed description to follow when read in conjunction with the drawings.